The present invention relates to methods of forming integrated circuits and circuits formed thereby, and more particularly to methods of forming integrated circuit capacitors and capacitors formed thereby.
The demand for higher capacity semiconductor memory devices has resulted in improved techniques to form memory devices and structures therein at higher levels of integration. However, because higher levels of integration typically require memory devices having smaller unit cell size, the area occupied by a cell capacitor in a memory device, such as a DRAM device, may have to be reduced significantly. As will be understood by those skilled in the art, this reduction in cell capacitor area can degrade memory cell performance at low voltages and adversely impact soft-error rate (SER) caused by xcex1-particle radiation.
Conventional methods of increasing cell capacitor area include forming cell capacitor electrodes (e.g., storage electrodes) with hemispherical grain (HSG) silicon surface layers. For example, a conventional method of forming HSG silicon surface layers on cell capacitor electrodes is disclosed in U.S. Pat. No. 5,407,534 to Thakur. However, while capacitors having HSG surface layers therein (hereinafter xe2x80x9cHSG capacitorsxe2x80x9d) have manifested enhanced capacitance in high density integrated circuits, HSG capacitors may lack stability and may incur performance degradation over the lifetime of an integrated circuit memory device. Studies have shown that the capacitance of a conventional HSG capacitor can vary greatly with respect to the polarity of a voltage applied across the capacitor""s electrodes. In particular, when the voltage between the upper and lower electrodes of a HSG capacitor switches polarity from a positive value to a negative value and becomes reverse biased (during such operations as reading and writing operations), a significant drop in capacitance may be observed. For example, FIG. 2 illustrates a capacitance response curve of a conventional HSG capacitor when a voltage is applied across its upper and lower electrodes. As shown, the maximum capacitance (Cmax) is obtained when the potential difference across the electrodes is positive. Yet, when the potential difference is driven to a negative value, the capacitance gradually drops. In fact, at a negative value of xe2x88x921.5V, the capacitance is at a minimum (Cmin), reaching only about 55% of Cmax.
It is therefore an object of the present invention to provide improved methods of forming integrated circuit capacitors and capacitors formed thereby.
It is still another object of the present invention to provide integrated circuit capacitors having electrodes with increased surface area, and capacitors formed thereby.
It is yet another object of the present invention to provide methods of forming integrated circuit capacitors having uniform capacitance characteristics when reversed and forward biased, and capacitors formed thereby.
It is still another object of the present invention to provide methods of forming integrated circuits having capacitors therein with improved long-term reliability, and capacitors formed thereby.
These and other objects, advantages and features of the present invention are provided by methods which include the steps of forming a lower electrode of a capacitor by forming a conductive layer pattern (e.g., silicon layer) on a semiconductor substrate and then forming a hemispherical grain (HSG) silicon surface layer of first conductivity type on the conductive layer pattern. The inclusion of a HSG silicon surface layer on an outer surface of the conductive layer pattern increases the effective surface area of the lower electrode for a given lateral dimension. The HSG silicon surface layer is also preferably sufficiently doped with first conductivity type dopants (e.g., N-type) to minimize the size of any depletion layer which may be formed in the lower electrode when the capacitor is reverse biased and thereby improve the capacitor""s characteristic Cmin/Cmax ratio. A diffusion barrier layer (e.g., silicon nitride) is also formed on the lower electrode and then a dielectric layer is formed on the diffusion barrier layer. The diffusion barrier layer is preferably made of a material of sufficient thickness to prevent reaction between the dielectric layer and the lower electrode and also prevent out-diffusion of dopants from the HSG silicon surface layer to the dielectric layer. The dielectric layer is also preferably formed of a material having high dielectric strength to increase capacitance.
According to a preferred aspect of the present invention, the step of forming a HSG silicon surface layer comprises seeding an upper surface of the conductive layer pattern with silicon seed crystals and then growing the seed crystals as single crystal grains. Steps are also performed to anneal the conductive layer pattern and then dope the HSG silicon surface layer with N-type dopants provided by a phosphine gas source. This doping step may be performed in a rapid thermal processing (RTP) apparatus and is preferably performed so that the HSG silicon surface layer has an N-type conductivity which exceeds the N-type of a portion of the conductive layer pattern extending adjacent the semiconductor substrate. This higher conductivity inhibits the formation of a depletion layer in the lower electrode when the capacitor is reverse biased. The diffusion barrier layer may also be doped in-situ with first conductivity type dopants to further prevent any reduction in the conductivity of the HSG silicon surface layer caused by out-diffusion of dopants to the dielectric layer. In addition, the diffusion barrier layer may be formed as a composite of a first silicon nitride layer formed by rapid thermal nitridation (RTN) and a second silicon nitride layer formed by chemical vapor deposition (CVD). The dielectric layer may also be formed of a high dielectric material such as tantalum oxide. In particular, the dielectric layer is preferably formed by forming multiple thin layers of tantalum oxide and then densifying each of the layers individually to improve the characteristics of the dielectric layer and underlying silicon nitride diffusion barrier layer.